The long-term goal of this project is to understand the molecular basis of synaptic specificity within the nervous system. Cellular manipulations in both vertebrates and invertebrates have shown that proper synaptic connectivity is achieved by neurons actively recognizing their appropriate targets. The molecular basis of this recognition is poorly understood, but it is hypothesized that there is some underlying system of unique molecular identities that allow neurons to distinguish their correct targets. This project will develop the Drosophila neuromuscular system as a model for understanding the molecular basis of synaptic specificity. To this end, we have identified and cloned a gene, apterous (ap), which is expressed in a small subset of developing muscles and neurons and controls the arborization pattern of motorneurons over the muscle subset. The experiments in this project will use molecular genetic, transformation, and histological techniques to: determine if the ap-expressing neurons are motorneurons to the muscles and whether they are altered in their innervation of the muscles in ap mutants; test the dependence of the ap motorneuron arborization phenotype on the lack of ap expression in the CNS and muscles. The hypothesis that individual muscles and motorneurons have unique molecular identities will be tested by identifying and studying genes expressed by subsets of the ap muscles and their motorneurons. For these studies a novel enhancer trap vector constructed in the lab which targets beta-galactosidase to axons will be used. These studies will provide us with a better understanding of the molecular basis of neuromuscular specificity in Drosophila should lead to an understanding of molecular mechanisms underlying synaptic specificity in other nervous systems, including those of higher vertebrates.